1. Technical Field
The present invention relates to an optical filter, an optical filter module, a spectrometric instrument, an optical instrument, and the like.
2. Related Art
An optical filter, such as an interference filter, is heretofore used in a spectrometric instrument or the like. As one form of the interference filter, a Fabry-Perot Etalon interference filter (hereinafter, referred to an Etalon filter or simply referred to as an Etalon) is known. For example, JP-A-11-142752 describes a transparent wavelength-variable interference filter (hereinafter, referred to as a variable gap Etalon filter or simply referred to as a variable gap Etalon) with a variable transparent wavelength. This interference filter includes a pair of substrates, and optical films (reflecting films) which are formed on the pair of substrates to face each other at a regular-interval gap. In this interference filter, the size of the gap between the optical films (reflecting films) changes due to external force to variably control the transparent wavelength.
In order to realize an Etalon having the desired characteristics, it is necessary to appropriately design optical films having a transmissive characteristic and a reflective characteristic with respect to light in a desired wavelength band. In general, many optical films have a structure in which the thickness of an optical film with respect to the wavelength λ of light is λ/4, and the optical films having the same structure are arranged to face each other and used in pairs. In order to realize more distinctive optical characteristics, the thickness of each optical film may have a value other than λ/4 (for example, JP-A-2008-129504). In JP-A-2008-129504, the optical films having the same structure are used in pairs.
In the Etalon filter serving as an optical filter, the filter characteristics (reflective characteristic, transmissive characteristic, and the like) are determined by the reflectance of the optical film. When the reflectance of the optical film is high, the half-value width becomes small, and when the reflectance of the optical film is low, the half-value width becomes large. By controlling the reflectance of the optical film, it is possible to control the filter characteristics of the Etalon filter. If the half-value width of the Etalon is small, only a desired wavelength is extracted. For this reason, when the Etalon is used as a spectrometric instrument, it is possible to increase measurement precision. However, if the half-value width of the Etalon is small, since the quantity of light which transmits the Etalon is lowered, the light-receiving unit may not easily detect light. Meanwhile, if the half-value width of the Etalon is large, the quantity of light which transmits the Etalon increases. In this case, the light-receiving unit easily detects light and also detects light other than a desired wavelength. That is, the wavelength resolution of the Etalon is lowered. When designing the Etalon, it is necessary that each of the “half-value width” and the “transmitted light quantity” satisfies a desired reference. Incidentally, for example, when the wavelength band of light to be used spans a wide wavelength range, it may become difficult to design the optical film.
As described above, the factor which determines the half-value width of the Etalon is the reflectance of an optical film. As a general method of controlling the reflectance of the optical film, a method is known which changes the material of the optical film or forms an optical film using a multilayer film and changes the number of layers in the multilayer film. However, when the material changes, there is a limit on the types of usable materials. When the number of layers of the optical film changes, the reflectance has only a discrete value with respect to the number of layers, and accordingly, it may be impossible to design an optical film such that a target reflectance, that is, a target half-value width or transmitted light quantity is obtained.
That is, when the number of layers changes, the value of the half-value width changes discretely, and a value other than the discrete value may not be realized only with a change in the number of layers. For example, when a specification is satisfied such that a half-value width is 3±0.5 nm, if discrete reflectance obtained with a change in the number of layers does not fall within the desired range in an optical film made of a predetermined material, it becomes difficult to appropriately design the optical film.